1. Field of the Invention
The present invention relates to an apparatus for roll-forming seamless annular products such as wheel rims of an aluminum alloy.
2. Description of the Prior Art
Known methods and apparatus for forming seamless rims for use on bicycles and motorcycles have been described, for example, in Japanese Laid-Open Pat. Nos. 43868/1974 and 42065/1976. Such apparatus generally comprise a pair of opposed rolls. A blank ring is sandwiched between the rolls as they are moved toward each other, to thereby enlarge the diameter of the blank ring. The ring is roll-formed into a rim, and there is provided a plurality of guide rolls for limiting expansion of the diameter of the blank ring.
The prior art apparatus are disadvantageous in that since plastic deformation of the blank ring is much greater in the radial direction than in other directions, the diameter of the blank ring as it is being processed tends to exceed a predetermined dimension before the blank ring is shaped into required cross-sectional contours. The guide rolls are effective to prevent such excessive expansions in diameter, however, they fail to give finished rims a high degree of circularity and dimensional precision because the guide rolls are angularly spaced from each other. With such guide rolls, there have sometimes been produced distorted rims. Various attempts have been proposed to eliminate such difficulty. One such effort has been to provide additional guide rolls. However, this has made the apparatus more complex in structure. Other attempts have involved additional devices or processing steps for correcting the finished rims, thus raising costs. Moreover, the rims thus formed are likely to have reduced strength resulting from the additional correction steps to which the rims have been subjected.
To solve the above-described problems with the conventional methods and apparatus, there has been proposed a roll-forming method comprising the steps of placing a blank ring between a rotatable annular die and a rotatable pressure roll disposed within and movable radially relative to the annular die, rotating the die or the roll so as to cause the other to revolve around the blank ring interposed therebetween, and moving the roll radially toward the die, thereby forming the blank ring into a predetermined cross-sectional shape as the ring is confined within the die. Such method is described in copending U.S. patent application Ser. No. 955,992 filed by the present applicant on Oct. 30, 1978 and entitled "Method of Manufacturing Seamless Wheel Rims."
According to such method, the blank ring is roll-formed by the annular die as the ring is confined within the die, with the results that an annular product is formed with precision as to outer diameter, cross-sectional shape, and circularity, and is free from distortions. The seamless annular product thus produced is required to be subjected only to a minimum number of finishing processes such as a process of removing burrs, with no correction process being required. The method is thus highly advantageous in that it can produce seamless annular products or rims easily, speedily, at less cost, and at a high rate of production.
It can be readily understood that an apparatus for carrying out such method should be able to roll-form annular products easily and reliably, be simple in structure and operation, and be as small in size as possible. Particularly, the annular die of an apparatus for forming annular products such as motorcycle wheel rims is of necessity great in size. Such an apparatus therefore has a number of attendant problems with regard to structure and arrangements for rotatably supporting the annular die and the pressure roll, and cylinder units.
There are a variety of requirements to be met in the construction of the above-mentioned apparatus. More specifically, the die for forming large-diameter products such as motorcycle wheel rims as well as mechanisms ancillary to the die become necessarily large in size, and hence it is difficult to move the die back and forth and up and down without excessive effort. Thus, instead of displacing the die, the pressure roll should be rotated, and moved to and fro, and up and down, and be utilized to load blank rings into position. The force with which the roll presses the blank ring against the annular die is considerably large since the blank ring is required to be subjected to cold rolling. The roll, which is rotatable and movable back and forth, is carried by a support which is itself movable back and forth. Mechanisms for the roll, including the roll support, are necessarily large in weight, and the overall weight of the mechanism and the force applied to the roll must be borne together. This results in a large cylinder unit for the roll, which then requires that arrangements supporting and mounting the cylinder unit be large in size and sufficiently rigid.
To enable removal of a formed product from the die, the annular die should be constructed of separable segments that can be assembled together and spread and separated from each other by a suitable mechanism. One such mechanism includes a cylinder unit which is axially actuated to clamp and release the die segments. With the cylinder unit axially arranged relative to the annular die, however, the die becomes exceedingly long, resulting in an increase in the overall size of the apparatus.
Further, operation of the mechanisms for the die is stopped each time a formed annular product is to be removed from the spread die segments, and is started again for a next cycle of roll-forming operation. Such a procedure involves a great loss of energy because the die is large in size and weight, with the result that the apparatus will operate at a reduced degree of efficiency and at a low rate of production.
In view of the problems attendant the prior art methods and apparatus, and further in view of the practical requirements to be satisfied in an apparatus for carrying out the method discussed above, the present invention provides an apparatus which eliminates the deficiencies conventionally experienced and meets the requirements set forth above.